The solid electrolytic capacitor using the conductive polymer as a solid electrolyte is less likely to ignite and has a low ESR (equivalent series resistance), compared with conventional solid electrolytic capacitors using manganese dioxide, a solid electrolyte. Thus, in view of various excellent characteristics, the market has been expanded rapidly.
The conductive polymer as mentioned above is generally manufactured by a chemical oxidative polymerization method. For example, a transition metal salt of an organic sulfonic acid such as iron p-toluenesulfonate is used as an oxidizer dopant solution, and a monomer such as thiophene or its derivative is polymerized. See patent publications Nos. 1 and 2 identified below.
Although this method was fit for mass production application, there was a problem that the transition metal used as an oxidizer could remain in the conductive polymer. Even if a washing process is performed in order to remove the transition metal, the transition metal is one having a character which is difficult to be removed completely. Thus, there was a request to improve the stability of a conductive polymer and a long-term reliability of a solid electrolytic capacitor by removing the influences of the transition metal on the conductive polymer or on the solid electrolyte capacitor in case where the transition metal remains in the conductive polymer. Therefore, there was a proposal to use an oxidizer made of one other than transition metal salts; i.e., peroxides, for example. However, there was still a problem that compared with the transition metal salts, the reactivity or the electric conductivity of the obtained conductive polymer was significantly low.
[Patent document 1]: Japanese Laid Open Patent Publication No. 10-50558, and [Patent document 2]: Japanese Laid Open Patent Publication 2000-106331.